Surface hardening of ferrous metals



' boron hydrldes.

Patented Jan. 10, 1950 SURFACE HABDENING F FERROUS METALS Herman I. Schlesinger and George W. Schaelfer,

Chicago, Ill.,

assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing.

Application November 28, 19, Serial No. 712,263

2 Claims. (Cl- 148-635) This invention relates to the art of surface hardening ferrous metals and more particularly the present invention relates to producing boroncontaining deposits on the surface of ferrous metals by the decomposition of gaseous diborane.

In accordance with the present invention, it has been found that an extremely hard boroncontaining layer is formed on the surface of ferrous metals, such as, for example, iron, steel, cobalt, or nickel, by heating the metal in an atmosphere of gaseous diborane at a temperature above 500 C. but substantially below the melting point of the metal. It has been found that in this range of temperatures ferrous metals react with diborane to give metal borides. The boroncontainin layer may be formed either by heating the article to be hardened in a static atmosphere of gaseous diborane or by heating the article while flowing diborane thereover.

A further feature of the present invention is the control of the pressure of the gaseous diborane which is used in forming the hardened layer on the surface of the ferrous metal. It has been found that it is desirable to perform the hardening operation at subatmospheric pressures preferably below about 50 millimeters. At higher pressures, non-volatile boron hydrides are formed which will accumulate on cool surfaces in the apparatus and thus diminish the yield of hardened metal from a given amount of gaseous diborane. It has also been found that the size of the object to be hardened is also important in determining the optimum pressure at which the process may be carried out. With larger objects, lower pressures should be used since here the gaseous diborane is in contact with the heated surface for a longer period of time and thus there i more danger of the formation of non-volatile When the hardening is carried out at about 700 C., pressures below 30 millimeters of mercury are found to give the most satisfactory results. It is not desirable to use extremely low pressures, however, since under these conditions the rate of hardening becomes so low as to be impractical.

In the process of the invention, the hardening effect is believed to be produced by the reaction between boron formed from the decomposition of diborane and the surface of the ferrous metal, and also, at least partially, to a direct reaction of the metal and diborane. This reaction results in the formation of an extremely hard layer of metal boride. Furthermore, it has been found that this surface layer of boride is extremely resistant to acid and alkaline corrosion.

In a preferred embodimentof this invention an iron cylinder may be hardened by treating it with diborane at 700 0. Prior to the coating operation the surface to be coated is thoroughly cleaned, as by machining or sandblasting and is degreased with a suitable solvent such as ethylene dichloride or carbon tetrachloride. The cylinder is then placed in a decomposition chamber and is degassed by induction heating to a dull red heat at an absolute pressure of 10- millimeters of mercury. The chamber is then filled with purified hydrogen to a. pressure of 20 centimeters, the cylinder is reheated to a dull red heat for about 10 minutes, and gases in the chamber are pumped out while the cylinder is still hot. The latter step is repeated until no more water is formed or until the gas removed contains no appreciable content of oxygen. The cylinder is then degassed until a pressure of 10- millimeters is attained within the chamber at 500 C., and remains at that pressure without further evacuation of the chamber. After the cylinder has been cleaned and deoxidized by the above-described method, the temperature is adjusted to 700 C. and diborane is passed through the chamber at a pressure of about 3 millimeters of mercury until a layer of boride about 5 microns thick is formed. This layer is extremely hard and resistant to abrasion. It is unaffected by an ordinary steel file, the teeth of the file being completely worn away.

It will be apparent that the above-described embodiment of the invention is illustrative, and many modifications thereof will be apparent to those skilled in the art without departing from the spirit of the invention or the scope of the appended claims.

The deposition of boron in amorphous form is the subject matter of our co-pending application, Serial No. 708,446, filed November '7, '1946.

What is claimed is:

l. The method of producing a hardened surface on ferrous metal which consists in contacting the surface of said ferrous metal and heating face on ferrous metal which consists in contact- (References on following page) 3 REFERENCES CITED The following references are 0! record in the file 0! this UNITED STATES PA'IENTS OTHER REFERENCES- Mellor, A Comprehensive Treatise on Inorganic.

and Theoretical Chemisty. vol. V (1924), pages 5 36 and 37, published by Lonzmans, Green and Co.,

New York.

Schlesinger, Schaefler and Barbaras, Document MDDC-1338, 28 pages; published September 19, 1947, U. S. Atomic Energy Commission, Oak Ridge,

1 Tennessee. 

1. THE METHOD OF PRODUCING A HARDENED SURFACE ON FEROUS METAL WHICH CONSISTS IN CONTACTING THE SURFACE OF SAID FERROUS METAL AND HEATING IT TO ABOUT 700*C. WITH GASEOUS DIBORANE AT SUBATMOSPHERIC PRESSURE OF BELOW 30 MILLIMETERS OF MERCURY WHEREBY METAL BORIDE IS FORMED. 